It weighs 500,000 pounds, more than the biggest 747. At 385 feet, its wingspan is longer than a football field. And now the giant Stratolaunch — the largest plane ever built and Microsoft co-founder Paul Allen’s project to lift rockets into the stratosphere before they are launched into space — is on the move. In video released by Allen, the twin-fuselage behemoth can be seen barreling down a runway at California’s Mojave Air & Space Port, reaching a speed of 46 mph in a taxiing and ground-handling test. ‘Captured new video of @Stratolaunch plane as it reached a top taxi speed of 40 knots (46 mph) with all flight surfaces in place on Sunday,” Allen wrote in a statement. “The team verified control responses, building on the first taxi tests conducted in December.” According to a report from NBC News, the plane is powered by six Pratt & Whitney turbofan engines and is intended to carry up to 550,000 pounds to an altitude of 35,000 feet. The plan is to suspend rockets between its two fuselages, where they can be more efficiently launched than from an earthbound pad. Space entrepreneur Gary Hudson told NBC that he thinks it has a good chance of success — in part because of Allen’s deep pockets.

“If they choose wisely, they could provide crew and light cargo services to LEO (low-Earth orbit) for considerably less than current prices,” he says. “If they don’t, they will become a footnote to astronautical history for having built the world’s largest plane,” referring to Howard Hughes’ famed Spruce Goose of 1947, which flew only once before being mothballed. According to the Tech Times website, the Stratolaunch is due for its maiden flight next year, which would make it the largest-ever plane to lift off, beating out Hughes’ huge seaplane. Hudson told Tech Times that launching rockets from high altitude has key advantages, including less atmospheric drag. Also, while a ground-launched rocket may have to wait to coincide with an orbiting spacecraft, the Stratolaunch could go to a launch point at any time.

Wingspan comparison of the Stratolaunch carrier with other large airplanes

This graphic shows the typical flight plan for a Stratolaunch mission. The plane would head out over water, as much as 1,150 miles from its base, and release a rocket that would then fire up its engine and continue on to orbit. The plane, meanwhile, would return to base. (Credit: Vulcan Inc.)